[GreenKeys] Dual head model 14 TD photos

[Don Robert House]

Thank you Jim,
This is fascinating! Thank you for taking time to write this out.
Best,
Don

On 7 Nov 2012, at 3:53 PM, Jim Haynes wrote:

On Wed, 7 Nov 2012, Don Robert House wrote:

> Thanks Jim,
>
> I always appreciate your input.  Did the military commonly use  
> multiplex circuits?
>
I don't know much about that at all.  I suspect the dual-headed TD was
someone's clever idea in WW-II to get more speed out of existing  
machines
and circuits.  What I have seen mentioned is running 15s at 100 wpm,
saying the excessive maintenance was justified by the war emergency need
to handle traffic faster.  But I don't recall reading anything about the
cheap two-channel multiplex.  The Army manual TM 11-2222 is dated 1945,
and a later version dated 1951.  At any rate, ya gotta admit it is
clever.

The Navy used Western Union time-division multiplex in WW-II.  That is
documented in Western Union Technical Review for January 1948.  I'm just
guessing that it might be because a Western Union official was taken  
into
the Navy during the war that they did it.  Otherwise it's hard to  
imagine
that early 1900s technology being used on ships and radio circuits.

Then Teletype had a time-division multiplex in its product line in the
late 1930s.  This must have been a hold-over from the days before AT&T
bought the company, because under AT&T ownership issues of transmission
were on AT&T turf and Teletype wasn't expected to be involved in them.
So I guess railroads and other private-line users were the users of the
Teletype multiplex product line, while W.U. had its own designs and
manufacture.

In the late 1930s Teletype hired an engineer to work on an electronic
version of the multiplex.  I don't know how this came about since it
probably would not have been supported by AT&T, but would have been
something Teletype had to do on its own.  This work was not completed
until after WW-II when Teletype developed the AN/FGC-5 four-channel
electronic multiplex (using vacuum tubes and thyratrons).  The first
customer was the Navy, presumably because they were already using the
old technology time division multiplex more or less successfully.
Eventually most or all of the military services and the C.A.A. made some
use of it.  It took two seven-foot racks to hold the four-channel
transmitter and receiver.  I know it was used on radio circuits because
I heard the signals, and decoded them with a multiplex under development
in the Teletype lab.

In the late 1950s Teletype developed a transistorized version of the
four-channel multiplex, AN/UGC-1, which was small enough to go into a
submarine.

I assume there was no non-government market for this stuff since it was
built to military standards and cost accordingly.

Meanwhile the military was moving into frequency division multiplex
(where AT&T and W.U. had already been for quite some time).  It was
standard to put 12 or 16 channels on one voice-grade circuit.

Teletype's last product in that line was the AN/UGC-3 sixteen-channel
multiplex, which occupied a seven-foot rack.  You wouldn't expect 16
channel time-division to work very well on HF radio channels because the
bits are so short and propagation tends to clobber short bits.  The
16-channel mux could be operated with 4 channels interoperating with
the 4-channel mux, or it could run 8 channels.  It was a long time
before was told the reason for this product.  Turns out they had a
crypto machine that could encrypt the 16-channel signal.  Then there
was a thing called the AN/UGA-1 which turned the 16-channel signal back
into sixteen synchronous single channels, which were transmitted on a
frequency-division system, and the whole process reversed at the  
receiving
end.  So the main purpose of the setup was to have one fast crypto  
machine
instead of sixteen slow ones.

One offshoot of the tube-type multiplex was a thing called the monoplex
because it was only a single channel.  It used quite a few parts of the
AN/FGC-5, converting the start-stop signal to a synchronous signal and
back again.  This was used by AT&T for the DEW-line communications where
it gave better performance than start-stop could.  I've been told the
circuit quality was good enough that they eventually used two-channel
multiplex on that system.